Recently, many proposals of applying photonic crystals to semiconductor lasers have been reported. Japanese Patent Laid-Open No. 2000-332351 discloses a light source employing a surface emitting laser in which an active layer including a light-emitting material is provided and a two-dimensional photonic crystal is formed in the vicinity of the active layer. The disclosed photonic-crystal surface emitting laser is one type of Distribution Feedback (DFB) laser and has a resonance mode in the in-plane direction of a substrate. In the two-dimensional photonic crystal, columnar holes are periodically formed in a semiconductor layer, and a distribution of its refractive index has two-dimensional periodicity. With the two-dimensional periodicity, part of light generated in the active layer, which has a particular wavelength, resonates to form standing waves, thus causing laser oscillation. Further, the light is taken out in a direction perpendicular to the laser surface with first-order diffraction, and a thus-obtained laser device operates as the surface emitting laser.
Trial products of the photonic-crystal surface emitting laser have been so far fabricated by using various compound semiconductors. Photonic-crystal surface emitting lasers using nitride semiconductors have also been studied.
When trying to reduce the cost of a nitride semiconductor laser, it is advantageous to employ an inexpensive heterogeneous substrate, e.g., a sapphire substrate, instead of an expensive GaN substrate. However, if crystal growth is developed on a heterogeneous substrate as it is, a difficulty arises in obtaining a high-quality crystal less subjected to transitions. In view of such a difficulty, the so-called ELO (Epitaxial Lateral Overgrowth) technique is often utilized for an improvement of crystal quality. With the ELO technique, some concave-convex structure is formed on a substrate or in a compound semiconductor layer to develop crystal growth in the lateral direction, thereby obtaining a crystal less subjected to transitions.
Japanese Patent Laid-Open No. 2000-021789 discloses a structure in which the ELO technique is applied to an edge emitting semiconductor laser.
When the ELO technique is applied to the photonic-crystal surface emitting laser, the following problem occurs which is not caused with the edge emitting laser disclosed in the Japanese Patent Laid-Open No. 2000-021789.
FIG. 11 is a schematic view to explain the structure of the edge emitting semiconductor laser disclosed in the above-cited Japanese Patent Laid-Open No. 2000-021789. In FIG. 11, reference numeral 1110 denotes a substrate, 1120 denotes a mask for selective growth, which is provided to implement the ELO technique, and 1150 denotes an active layer. In the edge emitting laser illustrated in FIG. 11, a resonance direction 1192 of laser light and emergent light 1190 are both restricted in the in-plane direction of the substrate 1110, and no light is emitted in a direction toward the substrate. Therefore, optical characteristics of the selective growth mask 1120 used in the ELO technique do not affect characteristics of a laser device.